Leibfried, in U.S. Pat. Nos. 4,900,779 and 4,902,731 and U.S. patent application Ser. Nos. 07/419,429 (filed Oct. 10, 1989, now U.S. Pat. No. 5,013,809) and 07/419,430 (filed Oct. 10, 1989, now U.S. Pat. No. 5,077,134) and Bard and Burnier, in U.S. patent application Ser. No. 07/422,214 (filed Oct. 16, 1989, now U.S. Pat. No. 5,008,360), describe cross-linked organosilicon polymers and cross-linkable organosilicon prepolymers comprised of polycyclic hydrocarbon residues and cyclic polysiloxanes or siloxysilane residues linked through carbon to silicon bonds, and processes useful for preparing the same. Cowan, in U.S. Pat. No. 4,877,820, discloses crosslinked or crosslinkable linear poly(organohydrosiloxane) polymers having at least 30% of their .tbd.SiH groups reacted with hydrocarbon residues derived from polycyclic polyenes. The crosslinked polymers have high glass transition temperatures (T.sub.g), low dielectric constant, low moisture absorption and other desirable properties. The polymers and prepolymers are described as useful for electronic applications, such as preparing printed circuit boards (including substantially tack-free prepreg and laminates useful for preparing such circuit boards) and encapsulants, and structural materials.
One major weakness of these polymers is their brittleness. Brittleness can result in cracking.
The inventors have discovered that certain elastomers toughen the polymers and prepolymers, without significantly impacting other properties, such as Tg.